Changes to Lucene are only persisted to disk during a Lucene commit,
which is a relatively heavy operation and so cannot be performed after every
index or delete operation. Changes that happen after one commit and before another
will be lost in the event of process exit or HW failure.

To prevent this data loss, each shard has a transaction log or write ahead
log associated with it. Any index or delete operation is written to the
translog after being processed by the internal Lucene index.

In the event of a crash, recent transactions can be replayed from the
transaction log when the shard recovers.

An Elasticsearch flush is the process of performing a Lucene commit and
starting a new translog. It is done automatically in the background in order
to make sure the transaction log doesn’t grow too large, which would make
replaying its operations take a considerable amount of time during recovery.
It is also exposed through an API, though its rarely needed to be performed
manually.

The data in the transaction log is only persisted to disk when the translog is
fsynced and committed. In the event of hardware failure, any data written
since the previous translog commit will be lost.

By default, Elasticsearch fsyncs and commits the translog every 5 seconds if index.translog.durability is set
to async or if set to request (default) at the end of every index, delete,
update, or bulk request. In fact, Elasticsearch
will only report success of an index, delete, update, or bulk request to the
client after the transaction log has been successfully fsynced and committed
on the primary and on every allocated replica.

In some cases (a bad drive, user error) the translog can become corrupted. When
this corruption is detected by Elasticsearch due to mismatching checksums,
Elasticsearch will fail the shard and refuse to allocate that copy of the data
to the node, recovering from a replica if available.

If there is no copy of the data from which Elasticsearch can recover
successfully, a user may want to recover the data that is part of the shard at
the cost of losing the data that is currently contained in the translog. We
provide a command-line tool for this, elasticsearch-translog.

The elasticsearch-translog tool should not be run while Elasticsearch is
running, and you will permanently lose the documents that were contained only in
the translog!

In order to run the elasticsearch-translog tool, specify the truncate
subcommand as well as the directory for the corrupted translog with the -d
option: